Transformer tap changing mechanisms



June 18, 1935. o. P. M CARTY 2,004,998

TRANSFORMER TAP CHANGING MECHANISMS Filed Nov. 29, 1933 Inventor:

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Patented June 18, 1935 UNITED STATES TRANSFORMER TAP CHANGING MECHANISMS Orin P. McCarty, Pittsfield, Mass., assignor to General Electric Company, .a corporation of New York Application November 29, 1933, Serial No. 700,239

4 Claims.

, My invention relates to transformer tap changing mechanisms and to protective circuits particularly adapted for use therein. Varying conditions in the circuits connected to a transformer often make it desirable to change the ratio of voltage transformation in the transformer without interrupting its load and the general object of the invention is to provide improved and reliable mechanism for this purpose. A further object of the invention is to provide a protective 1 circuit to prevent damage to the mechanism in case of faulty operation thereof.

The invention will be more fully explained in the following description taken in connection with the accompanying drawing which shows diagrammatically an auto transformer provided with voltage ratio adjusting means arranged in accordance with the invention.

The transformer I0 indicated in the drawing is connected between a high voltage circuit II and a low voltage circuit I2, the transformer winding having a common section I3 and a series section I4. The series winding section I4 is provided with taps I5 distributed along it and connected to the fixed contacts I6 and I! of two switches I8 and I9 respectively. The switch I8 has two movable contacts 20 and 2I fixed to a common shaft but angularly displaced and insulated from each other. The switch I9 has two similarly arranged insulated and angularly displaced movable contacts 22 and 23.

One side of the high voltage circuit II is connected directly to the movable contact 2| and through an impedance 24 and a fuse 24' or other circuit interrupting device to the movable contact 20 of the switch l8. Any form of impedance 24 desired may be used and it is shown in the drawing as a simple, non-inductive resistance. One side of the low voltage circuit I2 and the inner end of the common winding section I3 are connected directly to the movable contact 23 and through a resistance'25 and a fuse 25' or other circuit interrupting device to the movable contact 22 of the switch I9. The two movable contacts of each of the switches I3 and I9 are normally positioned on the same. fixed contact IE or I! respectively. It will be apparent that any desired number of turns of the series winding section I4 may be connected by means of the switches I8 and Him either direction in series with the common winding section I3. Thus, up to the maximum number of turns in the series winding section I4, any number of turns desired may be connected in either direction in series with the common winding section I3. With the switches in the positions shown in the drawing the entire series winding section I4 is connected in series with the common winding section I3.

The two sets of movable contacts of the two switches I8 and I9 are operated by the two gears 26 and 21 of a double Geneva gear driven through a worm 28. and pinion 29 by a reversible motor 30. The driving element 3| of the double Geneva gear has a single driving pin 32 so arranged that one half revolution of the driving element 3| will rotate one or the other of the, gear wheels 26 and 21 to change the voltage ratio of the transformer.

The coil 33 of a contact-making voltmeter 34 is connected to an auxiliary winding 35 inductively coupled to the common winding section I3 of the transformer III. The movable contact 36 of the voltmeter is biased by a spring 31 to hold it in neutral position under the normal desired voltage conditions in the low voltage circuit I2 which will be considered as the secondary or load circuit, the circuit I I being the primary circuit.

Now, if the voltage of the circuit I2 drops, the movable contact 36 of the voltmeter 34 will be pulled against the back contact 38 and close a circuit which may be traced from one terminal of the winding 35 through the back contact 38 of the voltmeter, the winding 39 of a relay 40 and back to the other terminal of the winding 35. This will cause the relay 40 to close two sets of contacts 4i and 42. Closing the contacts 42 completes a circuit which may be traced from one ter-- minal of the winding 35 through a conductor 43,

.rest three brushes 41, 49 and 49. The remainder of the drum contactor 45 is conductive. As soon as the motor starts, the three brushes 41, 48 and 49 are short circuited by the contactor 45. The short circuited brushes 43 and 49 and the contacts 4I of the relay 49 which were closed by the voltmeter 34 complete a holding circuit which may be traced from one terminal of the winding 35 through the conductor 43, the brushes 48 and 49, the contacts II, the winding 39 of the relay 40, and the conductor 44 back to the other terminal of the winding 35. This holding circuit keeps the contacts 4i clo ed and thus assures a complete cycle of operation of the switches even though the voltmeter contacts 36 and 38 should break the circuit before completion of the cycle. The contactor 45 rotates with the driving member 3| of the Geneva gear so that the insulating section 46 of the contactor is brought under the contacts 41, 48 and 49 at .the end of each operating cycle of the Geneva gear and therefore of the switches l8 and Hi. This breaks the, holding circuit through the contacts 4| and returns control of the relay 39 to the contact making volt meter-34. If the change imposition of the switchhas adjusted the voltage of the transformer to its normal value, then the contacts 35 and 38 of the contact making volt meter will open, the relay 39 will be released to open its contacts 42 and the .motor will stop. If the transformer voltage is still below its normal value, then the contacts and 38 of the volt meter 34 will still be closed and maintain-current in the relay 3!! to start another switch operating cycle. Thus the switches l8 and I! cannot stop in any intermediate position but must complete each operating cycle after it is once started.

If the voltage of the circuit |2 rises, the movable contact 35 of the voltmeter 34 will be moved against the front contact 5|) of the voltmeter to close a circuit which may be traced from' one terminal of the winding 35 through the contacts 36 and 50 of the voltmeter, the winding 5| of a relay 52 and the conductor 44 back to the other terminal of the winding 35. This will cause the two sets of contacts 53 and 54 of the relay 52 to close with the result that the switches l8 and I9 will be operated in a direction to raise the voltage of the circuit I2, the contacts 54 controlling the motor 30 and the contacts 53 controlling a holding circuit through the winding 5| of the relay 52 to assure the completion of each cycle of operation of the switches l8 and IS. This operation is similar to that which has been described in connection with the relay 40 for raising the voltage of the circuit l2 and it will be easily understood without further description.

The two movable contacts of each of the switches l3 and I9 are moved very quickly from one fixed contact to the next by means of the Geneva gear drive. Referring to the switch l8,

2| leaves this fixed contact l5 and until it reaches the next contact l6 all of the primary current will flow through the resistance 24 with its fuse 24' and the movable contact 20. After the movable contact 2| reaches the next fixed contact Hi, the resistance 24 and its fuse 24 will be bridged across the two contacts l5; and will carry a circulating current due to the voltage between the transformer taps |5 to which the two contacts l6 are connected. An instant later the movable contact 20 will leave the first contact l5 and then all of the primary current of the transformer may flow directly through the movable contact 2|.

Under normal conditions there is very little or no current flowing through the resistance 24 and its fuse 24' except for a very brief instant while the movable contact 2| of the switch is moving between two fixed contacts Hi. In case of accidental breakage or faulty operation of some part of the switch operating mechanism, however, it is possible that the switch might stop with its movable contact 2| between two fixed contacts l6. Under this condition the resistance 24, if not protected, would have to be large enou 24 is-provided with a fuse 24' or other circuit interrupter and a cutout 55 is connected across the terminals of the resistance and fuse. This cutout may be of any suitable construction but it is indicated in the drawing as including two conductive plates 55 connected to the two terminals of the series connected resistance and fuse and separated by an insulating film 51 which will break down under a predetermined voltage across the cutout and thus short circuit the resistance and fuse. The cutout 55 is designed to withstand the voltage across the resistance and fuse during the very brief instant while the resistance is carrying full primary current under normal switch operating conditions. If this full primary current continues to flow through the resistance, however, due to faulty operation of the switch, the fuse 24' will blow and protect the resistance. The film 51 will then' instantly break down and permit the current toflow and the load circuit to be supplied with energy without interruption.

A cutout 58 is similarly connected across the resistance 25 and fuse 25' or other circuit interrupter associated with the movable contacts 22 and 23 of the switch IS. The purpose and function of this cutout 58 and fuse 25' are the same a particular form of transformer and control mechanism but it will be obvious that it may be otherwise applied without departing from the 'spirit of the invention and the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

1. The combination with a transformer winding provided with taps, of a switch having fixed contacts connected to said taps and two contacts insulated from each other and movable along said fixed contacts, means for moving said movable contacts alternately from one fixed contact to another, an impedance and a circuit interrupter connected in series between said movable contacts, and a normally open cutout connected in parallel with said impedance and interrupter, said circuit interrupter being responsive after a predetermined interval of time to open the circuit through said switch, whereby substantially full circuitvoltage is impressed across said cutout, and said cutout being responsive to said voltage to close said circuit.

2. The combination with a transformer winding provided with taps, of a switch having fixed contacts connected to said taps and two angularly displaced contacts insulated from each other and movable along said fixed contacts, an impedance an a circuit interrupter connected in series between said movable contacts, and a normally open cutout connected in parallel with said impedance and interrupter, said circuit interrupter being responsive after a predetermined interval of time to open the circuit through said switch, whereby substantially full circuit voltage is impressed across said cutout, and said cutout being responsive to said voltage to close said circuit.

3. The combination with a transformer winding provided with taps, of a switch having fixed contacts connected to said taps and two contacts insulated from each other and movable along said fixed contacts, means for moving said movable contacts alternately from one fixed contact to another, an impedance and a circuit interrupter connected in series between said movable I contacts and having substantially smaller current carrying capacity than that due to maximum normal transformer load, and a normally open'cutout connected in parallel with said impedance and interrupter, whereby substantially full circuit voltage is impressed upon said cutout upon opertion of said circuit interrupter, said cutout being responsive to said circuit voltage to break down and short circuit the two movable contacts,

4. In an alternating current circuit, an impedance, a circuit interrupting means in series with said impedance, and a normally open cutout connected in parallel with said impedance and circuit interrupting means, said circuit interrupting 

